One of the perks of using Android over iOS, is the fact that not every app is approved by Google (although that's changing in the near future), while Apple does approve every single app and update for iOS. When it comes to Android Auto, every app does need to be approved to be compatible with head units. And there's a good reason for that. It's not to keep malicious apps out - although that is likely happening too. The big reason is safety. Google wants your eyes to remain on the road, as they should be. Which is why only a handful of apps are compatible with Android Auto, namely media and messaging apps.
This might seem like an over-reaction, but think about it this way, if you download an app that can take over the head unit and possibly distract you more, that would not be a good thing. If Android Auto was a bit more open, we'd probably see Netflix appear on Android Auto, but that's not a good idea. While we'd love to watch Netflix in the car, it's just not safe. Google's Android Auto team has said that they are also limiting apps to those that would make sense in the car. Seeing as Twitter or Facebook wouldn't make sense or is needed in the car. But something like Spotify, or Hangouts would make sense in the car.
Now Google won't be limiting the apps that are available for Android Auto forever. But for now they are. They have also limited developers with the API for Android Auto. Google doesn't just check out the app to see what it does, but they physically test each app to make sure it works as it should. The last thing you'd want to happen when your driving is for Spotify, or Google Play Music Unlimited crashes when you're driving. That's not a good thing, primarily because it'll grab your attention, when your attention should be on the road - since we don't have self-driving cars just yet.
It's very Android-y of Google, but it's definitely worth it, at last for now. That'll change in the future, as we mentioned already.